Due in part to environmental considerations, current interest in the coating field has shifted from solvent-borne coatings, which have heretofore been used, to water-borne sprayable coatings and to water-based electrodepositable coatings. Electrodepositable coatings, especially cathodic electrophoretically depositable coatings, are of particular interest for high volume applications.
Several electrodepositable primers have been investigated for use on aircraft parts, to prepare such parts for the application of structural bonding adhesives and to inhibit corrosion. These primers include the anodic electrodepositable primers described in U.S. Pat. No. 4,405,427 and the cathodic electrodepositable primers described in Diener, S.L. "Exploratory Development of Corrosion Inhibiting Primers", Report No. AFML-TR-77-71 (May, 1977), Diener, S.L. "Development of Improved Electrodeposited Corrosion Inhibiting Primers", Report No. AFML-TR-79-4073 (June, 1978), Diener, S.L. and Mels, S. J., "Electrodeposited Corrosion Inhibiting Adhesive Primers", Proceedings of the 11th National SAMPE Technical Conference, Azusa, California, page 759 (1979), Beckwith, G.E., "Interfacial Bond Integrity (350.degree. F Service)", Report No. AFWAL-TR-82-4171 (January, 1983) and Meade, L.E. and McBrayer, T.E., "Manufacturing Technology for Integration of Advanced Repair Bonding Techniques", Report No. IR-466-1(IX) (January 15, 1985).
The anodically-applied primer of the ,'427 patent would be expected to have only limited corrosion resistance. Adhesive bonds made with the primers of the other references cited above exhibit a loss in shear strength after exposure to elevated temperatures. For example, the best reported primer of the Beckwith reference was a composition identified as "MD-902". Using "AF-143" epoxy film adhesive (3M), this primer provided adhesive bonds that after exposure to 149.degree. C. for only 200 hours exhibited an 18% drop in lap shear strength. In comparison, bonds made using state-of-the-art solvent-borne primers and the same epoxy film adhesive typically exhibit an increase in lap shear strength when exposed to similar conditions, since the cured film adhesive gains strength when first heated.
Improved retention of adhesive strength at temperatures of 149.degree. C. and above and service times of 200 hours and longer is considered desirable, particularly for applications such as the manufacture or repair of aircraft, in order to insure that the bonded assembly will not fail in use. As indicated by the above references and data, current primers do not satisfy this need. In an effort to obtain primers that will satisfy it, the U.S. Air Force recently issued RFP No. F33615-86R-5009 for "Electrodeposited Primer Development".
It should be noted that there are many references relating in general to electrodepositable paints ("ED paints") and paint primers. These references include Wismer et al, "Cathodic Electrodeposition", Water-Borne & Higher Solids Coatings Symposium (New Orleans, Feb. 17-19, 1982) and U.S. Pat. Nos. 3,663,389, 3,935,087, 3,959,106 and 3,962,165. ED paints and paint primers sometimes contain bis-maleimide crosslinkers. References describing such crosslinkers include U.S. Pat. Nos. 3,925,181, 4,025,409, 4,029,561, 4,035,272, 4,037,018, 4,130,469 and 4,140,816. In general, these references employ a Michael addition reaction, and to enable crosslinking to occur they require the presence of mercaptan groups, primary or secondary amino groups, formation of a primary or secondary amino group after electrodeposition, or the presence of an ultraviolet (UV) photoinitiator and the use of a UV cure cycle.
None of the above references describe the water-compatible coating compositions of the present invention.